I Don't Know How She Does ItAllison PearsonChatto and Windus £12.99, pp354

Allison Pearson boasted, in a recent article, that four working mothers had already resigned from their jobs and two had become pregnant after reading her new book (and that's before publication). I am not sure why she feels responsible for their pregnancies but I approached I Don't Know How She Does It with cautious curiosity.

The book began as a weekly Daily Telegraph column about Kate Reddy, a fund manager trying to 'juggle' (an inaccurate, vain verb) children and career. Kate Reddy. A pity about her name, I thought, and the battery-operated puns attending it. It seemed to pitch Kate, before she had a chance to defend herself, into strip cartoon.

This is what Kate believes: if you are a working mother, you must pretend to be childless at work. Should you wish to attend a school nativity play, you must concoct a professional excuse before scuttling off. There may be some truth to this: I know two female executives working for the same firm who lied to attend their children's sports day. Later, on opposite sides of the playing fields, they spotted each other.

Pearson determines to tell the truth about sports days, nativity plays and all the other, more insidious, working-mother-lies. She removes the gag from our mouths with a tremendous flourish. In particular she studies the way we dissemble, deny and hide our more complicated maternal feelings (from bosses, husbands, nannies, children and ourselves). She shows that 'having it all' is sometimes distinct from being able to enjoy any of it. What she describes is painful. But it makes grandly self-indulgent reading.

This is a book that could only come from a privileged place. Allison Pearson and Kate Reddy have fantastic luck: a good job, full-time nannies, husbands who cook (nice pasta dishes). Above all, they have the chance to complain extravagantly, leaving no lurch of the heart unmonitored. And they can choose not to work. It was only as a grateful member of the book's target audience that I was able to love every self-pitying minute of it.

Pearson writes with instinctive comedy: against the clock, with pressured panache. This is a bag/housekeys/money/mobile/got to go-style that suits its dashing - and dashed - heroine. Her writing is fun whether she is serious, trifling or merely describing the trifle constructed by an ambitious mother as 'the size of an inverted Albert Hall'. At no point is this a dreary whinge from the kitchen... it is a comic broadcast from the office. Pearson's jokes have a manic, hyperbolic quality, laughter that might at any moment tip over into tears. And she is a dab (or dabbing) hand at sentimentality.

I found it strangely restful to read about frantic Kate, consoled by what I recognised and what I didn't. Pearson is unflinchingly accurate about relationships between mothers and nannies - controlled jealousy, suppressed criticism, qualified gratitude. She observes finely, too, the way working mothers are needily over-attentive to their children, compared to comfortably off-hand, permanent mothers, but mercilessly sends up 'Mother Superiors', whose children are their careers.

I am in sympathy with most of this. But I have never once baked - let alone faked - cakes in the small hours to impress any school. Kate, as the novel opens, is to be found inflicting GBH on Sainsbury's mince pies in the hope of giving them a dented, homemade look.

In an afterword, Pearson thanks her husband, Anthony Lane, for adding commas and semi-colons but the parentheses, I swear, are all her own work. Read Pearson for these anything-but-throwaway asides: 'Mysteriously, childcare, though paid for by both parents, is always deemed to be the female's responsibility.' Or: 'Is it coincidence that we spend far more than our parents ever did on the restyling and improvement of our homes - homes in which we spend less and less time... It is as though home had become some kind of stage-set for a play in which we one day hope to star.' Or: 'When I was younger, I wanted to go to bed with other people: now I have two children my fiercest desire is to go to bed with myself.'

There is a poignancy about the under-written account of the children in the novel. They are encountered, mostly at night, waking up to reclaim their mother. They suffer a version of attention deficit disorder (it's their mother's attention they lack). When describing children, Pearson cannot compete with Helen Simpson's Hey, Yeah, Right, Get a Life. The combination of passion and tedium is missing. Nor does she stop long enough to examine properly what staying at home might involve as Rachel Cusk does in her trapped lament, A Life's Work.

But then Allison Pearson is best at describing the away game (whereas Cusk and Simpson are under house arrest): Kate Reddy has a job, she just doesn't know what to do with it. One moment she says: 'I think giving up work is like becoming a missing person'; the next she is slavishly admiring a non-working wife. Pearson's book is not a manifesto, it is an ambivalent polemic, if such a thing is possible.

As fiction, it is an also-ran, and as a novel, it doesn't quite believe in itself - only Kate is fully realised; other characters strut into the book like amateur actors. Kate's father is a dull, contemporary version of Eliza Doolittle's dad, her mother doesn't survive characterisation. Kate's email romance with Jack Abelhammer doesn't altogether convince, although it lifts the novel - and Kate - into another place for a while.

'When I wasn't at work,' Kate Reddy complains, 'I had to be a mother, when I wasn't being a mother, I owed it to work to be at work. Time off for myself felt like stealing.' While racing to finish the book, in the evening, my children escaped into the garden in their pyjamas and started to fight fiercely. I could hear their yells as I reached Pearson's last sentence. But there is good news: I am not pregnant - and still working.